When I first ran with Clover Coyner, she was an enthusiastic 29-year-old who could run endless 7:40s and never met a hill she couldn’t beat. A year later, she was a minute-a-mile slower, walking hills, depressed, and soundly tired of well-meaning friends trying to give her motivational speeches. A series of major expenses had also left her strapped for cash. But, amazingly, she still wanted to run, so she decided to sell plasma to pay race entry fees.

In the process, the plasma center ran a blood test. "Did you know that your iron levels are low?" they asked. Not quite anemic, but right on the borderline. Coyner was stunned. And angry. She’d had a physical only a few months before, and the doctor hadn’t said anything about low iron. Rather, he’d checked a box marked "normal." That, she now realized, was technically correct. But not by much. And it explained everything.

Low iron isn’t just a women’s problem. Last summer, 2004 Olympic marathon trials winner Alan Culpepper had the same problem. "I felt really sluggish," he says. "One day, I did repeat 1,000s on the track. They weren’t terrible, but they were a struggle. When I finished, I sat down and felt like I could go to sleep, right after the workout. [So] I got my numbers checked." Culpepper wasn’t borderline anemic, but he wasn’t where he should be either. "I knew right away that that was the problem," he says.

Iron deficiencies take two forms. Full-blown anemia is a recognized and sometimes severe medical problem. Untreated, it can take you from peak performance to the point where walking feels like a marathon. Severe cases can be life-threatening. It usually develops over the course of weeks, but it can also come on suddenly due to a major problem, such as internal bleeding.

Low-normal iron levels aren’t viewed as symptoms of major illness by the medical establishment, but as Coyner discovered, they can still make you feel dead on your feet.

Unfortunately, the changes are insidious. "It’s not the first thing you think of," Culpepper says. "Unlike other blood disorders, where you can really tell something is not right, with iron it can sneak up on you. You attribute it to other things: weather, training, life."

Overtraining is the first thing coaches and astute runners are likely to assume. (The non-astute just try to power through it.) A big hint that the problem might be iron (or some other medical condition) is if you get a case of what British runners sometimes call "the plods" and the standard treatment of backing off on your training doesn’t work, even if you back off a second time. View this as a red flag that something isn’t normal.

Treatment is fairly simple. Coyner immediately knew both the problem and the solution. For several months, ending shortly before her blood test, she’d been living with her vegetarian sister. Coyner had let her cook, but didn’t think much about the nutritional complexities. "So I went out and bought a great big steak," she says. Today she’s back to attacking 1,000-foot hills and running endless 7:40s.

Culpepper consulted an expert and started taking iron supplements. It took six weeks for his performances to return to normal. "Once you get in a hole it takes a while to get out of it," he says.

Figuring out what went wrong can be more difficult. While Coyner’s case was obvious, Culpepper thinks he was bushwhacked by a combination of factors. One was living and training in Colorado. "It’s an issue that many athletes face at altitude," he says. Prior to his performance drop, he was also suffering from mild tendinitis: not enough to stop him from training, but enough to require being on anti-inflammatories for an extended period of time. These can contribute to iron loss via gastrointestinal bleeding. On top of all that, it was an extremely hot summer. "I was sweating a lot and losing electrolytes," he says. "I found out after the fact that you lose iron that way."

In the 2003 edition of TheLore of Running, South African exercise physiologist Timothy Noakes agrees. High-mileage runners may also be losing iron if repeated pavement pounding is mashing too many blood cells in the capillaries of their feet. Or they may simply not be absorbing as much from their intestines. High-intensity running can also cause blood cells to leak into the urine. One study, he says, has found that the average lifespan of red cells in runners is only two-thirds as long as in non-runners. All told, though, Noakes believes that runners generally develop anemia for the same reasons as anyone else.

The main difference with runners may simply be that they’re more attuned to changes that standard checkups would ignore. However, Kelly Scott, an M.D. and former collegiate soccer player warns, men and non-menstruating women shouldn’t rush out and buy iron supplements merely because they feel tired. A genetic condition called hemochromatosis can allow some people to accumulate too much iron. "Hemochromatosis is fairly common and can lead to liver failure, among other things, if people take more iron than they need," she says.

Coyner and Culpepper both have the same advice for other runners: track your iron levels. Coyner continues to sell plasma, not only because it pays her race fees, but because it gives free blood tests. She keeps the results in her training diary, where she finds a strong link to recent race performances.

Culpepper’s approach is a bit more palatable for the average runner. First, don’t settle for a simple statement of "normal" or "low" in your annual checkup, he says. Get a number, beginning now, by having your doctor fax you a copy of your most recent test. "Then start a file, so you can compare. That’s what I did, and my ferritin was lower than I’d ever seen in ten years of blood tests," Culpepper says.